As a method for industrial production of a vinyl chloride-based resin, generally employed is batch suspension polymerization, in which a vinyl-based monomer, such as vinyl chloride, is dispersed in an aqueous solvent in the presence of a dispersion stabilizer, and subjected to polymerization with use of an oil-soluble polymerization initiator. In a polymerization process, the quality of such a vinyl chloride-based resin is controlled by various factors, for example, the polymerization rate, the ratio of the aqueous solvent to the monomer, the polymerization temperature, the kind and the amount of the polymerization initiator, the type of the polymerization tank, the stirring rate, and the kind and the amount of the dispersion stabilizer. Among them, the dispersion stabilizer has a significant influence.
The role of the dispersion stabilizer in suspension polymerization for a vinyl chloride-based resin is to disperse a monomer in an aqueous solvent, to give stable droplets, to uniformize the sizes of the droplets repeating dispersion and adhesion, and to control the cohesiveness of polymerized particles. Accordingly, the dispersion stabilizer needs to have, for example, the following properties:    (1) to give vinyl chloride resin particles having a sharp particle size distribution,    (2) to give porous particles of the vinyl chloride-based resin having an excellent plasticizer absorption property and an excellent molding processability,    (3) to give vinyl chloride-based resin particles having a porosity within a certain range to facilitate the removal of residual monomers,    (4) to give vinyl chloride-based resin particles having a higher bulk specific gravity to improve the processability of the vinyl chloride-based resin, and the like.
That is, the required performances of the dispersion stabilizer are, in summary, to exhibit an excellent dispersion ability even when used in a small quantity, and to appropriately control the particle diameter, the particle form, etc. of a vinyl chloride-based resin.
Generally, as the dispersion stabilizer, polyvinyl alcohol-based resins (hereinafter, polyvinyl alcohol may be abbreviated as PVA), cellulose derivatives, and the like are used alone or in combination thereof as appropriate. In particular, PVA-based polymers are the most widely used. However, such materials do not sufficiently meet the above requirements of the performances, and various examinations have been continuously carried out.
For example, Non-patent Literature 1 discloses a method where a dispersion stabilizer for suspension polymerization of vinyl chloride is a PVA which has a viscosity-average polymerization degree of 2000 and a saponification degree of 88 or 80 mol % and which is considered to have a high emulsifying capacity, or a PVA which has a viscosity-average polymerization degree of 600 to 700 and a saponification degree of around 70 mol % and precipitates at the polymerization temperature for the vinyl chloride-based resin.
Moreover, Patent Literature 1 discloses a method where polyvinyl acetate obtained by polymerization in the presence of aldehydes is subjected to saponification to give a PVA, sodium acetate is added to the PVA, the mixture is subjected to heat treatment to give a PVA having a polymerization degree of 1500 or less, a saponification degree of 90 mol % or less, and a carbonyl group and two or three vinylene groups linking with one another in a molecule, and the PVA is used as a dispersion stabilizer for suspension polymerization of vinyl chloride.
Furthermore, Patent Literature 2 discloses, as a dispersion stabilizer for suspension polymerization of vinyl chloride, a specific PVA of which a 0.1% by weight aqueous solution shows an absorbance of a certain value or higher at a wavelength of 280 nm and 320 nm, the absorbance being an index showing the amount of vinylene group contained in the molecule, and of which the ratio of the absorbance at 320 nm to the absorbance at 280 nm is a certain value or greater.
However, the PVAs described in the literature (Patent Literature 1 and Patent Literature 2) are PVAs previously subjected to heat treatment, and when heat-treated PVAs are used in suspension polymerization of vinyl chloride, the effect is not always satisfying in view of polymerization stability. For improvement of polymerization stability, the use of PVA previously subjected to strong heat treatment is required, but strong heat treatment induces yellowing in PVA and therefore, the resulting vinyl chloride resin (PVC resin) is not good in hue. Moreover, the PVA has a high block character, leading to reduction in dispersion performance, and thus, a porous vinyl chloride resin is difficult to obtain.
In addition, the use of various kinds of modified PVA, such as an ethylene-modified PVA (Patent Literature 3), a PVA having 1,2-diol in a side chain (Patent Literature 4), a PVA having a hydroxyalkyl group having 1 to 20 carbon atoms (Patent Literature 5) as a dispersion stabilizer has been examined. However, also in cases where these dispersion stabilizers are used in suspension polymerization of vinyl chloride, the effect is not satisfying in view of polymerization stability.
Moreover, a PVA esterified with a carboxylic acid having an unsaturated double bond (Patent Literature 6) has been examined. However, also in cases where the PVA is used as a dispersion stabilizer for suspension polymerization of vinyl chloride, the effect is not satisfying in view of polymerization stability, due to the strong hydrophilicity of carboxylic acids and carboxylic acid esters.